A 68-year-old woman living in Jiangsu Province in eastern China, near Shanghai, had contact with live poultry before she began developing pneumonia-like symptoms December 25. She was admitted to a hospital January 1 and was discharged after successful treatment January 22.
The patient's close contacts were under "medical surveillance," and none developed signs or symptoms of illness. But health officials are asking the public to remain vigilant and to practice good hygiene.
"Based on the seasonal pattern, the activity of avian influenza viruses is expected to be higher in winter," the Centre for Health Protection said. Travelers to affected areas "should be alert to the presence of backyard poultry when visiting relatives and friends. They should also avoid purchasing live or freshly slaughtered poultry, and avoid touching poultry/birds or their droppings."
Most avian flu strains are not transmitted to humans. The H7N4 avian flu strain was first identified among chickens in Australia in 1997, according to the World Health Organization
. But this is the first known human case of this strain.
"Our surveillance mechanisms -- the mechanisms that we use to detect influenza strains and pick up these unusual strains -- is vastly more sophisticated now than it has been in the past," said Dr. William Schaffner, an infectious disease specialist at the Vanderbilt University School of Medicine
. "It's a testimony to the comprehensiveness of our surveillance."
According to the US Centers for Disease Control and Prevention
, there are four types of influenza viruses: A, B, C and D. Wild aquatic birds are the natural hosts for most influenza type A viruses, although two subtypes (H1N1 and H3N2) are circulating humans as well. Type B influenza viruses are present only in humans.
As with most influenza viruses, the H7N4 strain appears to spread by respiratory droplets that may be inhaled by people who spend a great deal of time around poultry.
"On occasion, this virus can get in and find receptors that are actually rather deep in the bronchial tubes and, once that happens, you can get illness, but it's rare," Schaffner said.
According to a statement from China's Ministry of Health, 28 people who had been in contact with the infected person were monitored and did not show any signs of infection.
"She's what we call a dead-end infection," Schaffner added. "It doesn't go anywhere from her."
Type A viruses can be further subdivided based on two proteins -- hemagglutinin (HA) and neuraminidase (NA) -- found on the surface of the virus. There are 18 known HA subtypes and 11 known NA subtypes, according to the CDC.
H7 virus infections in humans are not common and typically cause milder or more moderate illnesses than other avian strains, with symptoms such as conjunctivitis and upper respiratory infections, according to the CDC. The most frequently identified H7 strain strain in humans is H7N9, with the first human infections reported in China in 2013
According to the CDC, 1,565 people have been infected with the H7N9 strain since 2013; 39% have died.
But according to Schaffner, the H7N4 case probably poses no serious threat to humans.
"I think the risk is really zero at the present time," he said. "We ought to take some comfort that we, in collaboration with the World Health Organization, have a distant early warning system that is picking up these cases and monitoring them very, very closely.
"We watch all this very closely, because if one of these strains happened to be able to pick up the genetic capacity to be transmitted from person to person readily, we would be on the threshold of a new pandemic."
According to Schaffner, treatment for infection with avian flu strains typically includes symptom management, which can include fever-reducing medication, hydration and antiviral medications.
"At the moment, the illnesses that (avian flu strains) tend to cause seem to be more severe," he said. "But they still respond to prompt treatment."